First Report of Pythium Root Rot of Fennel in California Caused by Pythium sulcatum

From 2011 through 2014, commercial fields of fennel (Foeniculum vulgare, Apiaceae) in Monterey County, CA, exhibited symptoms of a previously undescribed disease. Affected plants, placed in the field as transplants, were stunted and grew poorly. Older leaves were chlorotic and later dried up. Feeder roots had discrete brown to orange lesions, or were entirely necrotic. Plants occasionally died; however, most plants remained viable but were stunted and delayed in development. Disease incidence ranged from <1 to 15%. Isolations from root lesions consistently resulted in the recovery of a Pythium species. Based on cox1 DNA sequence data, the isolates were identified as Pythium sulcatum (100% sequence identity with GenBank Accession No. HQ708878) (Robideau et al. 2011). Inoculum for pathogenicity tests was prepared by growing isolates on cornmeal agar in 100-mm plates for 5 days, then grinding up the mycelium and agar in 50-ml sterile distilled water. Ten milliliters of the mixture were poured into the root zone of potted, 1-month-old fennel plants. Three isolates were prepared in this manner and used to inoculate 10 plants each. Immediately after inoculation, all potted fennel plants were placed in individual shallow pans of water for 24 h to create waterlogged conditions. Plants were then removed from the pans and maintained at 24 to 26° C in a greenhouse. After 2 weeks, older leaves of inoculated plants turned chlorotic. Feeder roots exhibited root lesions similar to those observed on field plants. The isolates recovered from all root lesions were morphologically and genetically identical to the original isolates. Negative control plants, which were inoculated with a sterile agar and water mixture, did not show symptoms on leaves or roots. The inoculation was repeated and the results were the same. To our knowledge, this is the first report of a root rot disease of fennel caused by P. sulcatum in California. This field location has a crop history that includes numerous fennel plantings as well as lettuce and cauliflower. A Pythium root rot disease has been reported on hydroponic fennel grown in South Africa (Labuschagne et al. 2003). Pythium sulcatum has been reported on other Apiaceae crops such as carrot (Daucus carota), celery (Apium graveolens), and parsley (Petroselinum crispum) (Farr and Rossman 2015).